JP7680914B2 - Tripod type constant velocity joint - Google Patents

Description

The present invention relates to a tripod-type constant velocity universal joint used for power transmission in automobiles and various industrial machines.

In drive shafts used in automotive power transmission systems, a sliding type constant velocity universal joint is often connected to the inboard side (the center side in the vehicle width direction) of the intermediate shaft, and a fixed type constant velocity universal joint is connected to the outboard side (the end side in the vehicle width direction). The sliding type constant velocity universal joint here allows both angular displacement and relative axial movement between the two shafts, while the fixed type constant velocity universal joint allows angular displacement between the two shafts but does not allow relative axial movement between the two shafts.

Tripod-type constant velocity universal joints are known as sliding type constant velocity universal joints. There are single roller type and double roller type tripod-type constant velocity universal joints. The single roller type has a roller inserted into the track groove of the outer joint member, which is rotatably attached to the truss of the tripod member via multiple needle rollers. The double roller type has a roller inserted into the track groove of the outer joint member, and an inner ring that fits around the truss of the tripod member and supports the roller rotatably. The double roller type has the advantage that the roller can be oscillated relative to the truss, and therefore can reduce induced thrust (axial force induced by friction between parts inside the joint) and sliding resistance compared to the single roller type.

An example of a double-roller type tripod constant velocity universal joint is disclosed in the following Patent Document 1. In such a double-roller type tripod constant velocity universal joint, the outer peripheral surface of the truss of the tripod member and the inner peripheral surface of the inner ring are in point contact or near-point contact on the torque load side. Therefore, in this type of tripod constant velocity universal joint, the surface pressure at the contact area between the outer peripheral surface of each shaft and the inner peripheral surface of the inner ring becomes high, especially under high load torque. This may affect the durability of the contact area of the outer peripheral surface of the truss.

To solve this problem, the following Patent Document 1 discloses a double-roller tripod constant velocity joint in which a hardened layer is formed on the leg shaft by carburizing, quenching and tempering, the tripod members are made of steel with a carbon content of 0.23 to 0.44%, and the effective hardened layer has a limit hardness of 600 Hv.

JP 2020-106087 A

The double-roller type tripod constant velocity universal joint described in Patent Document 1 is obtained, for example, by carburizing and quenching chromium-molybdenum steel with a carbon content of 0.34%, followed by high-temperature tempering. With this configuration, the carbon content in the steel can be increased compared to conventional methods, so that even if the contact surface pressure at the contact area between the outer circumferential surface of the truss and the inner ring becomes high due to an excessive torque load, the durability of the truss at the contact area can be improved.

On the other hand, the inventors of the present application further studied the tripod-type constant velocity universal joint described in Patent Document 1 and found that while durability was ensured at the contact portion of the outer circumferential surface of the trunnion under torque load, as described above, there was a problem with the strength of the trunnion's base portion. A tensile load is repeatedly applied to the trunnion's base portion as torque is transmitted, but the fatigue strength of the base portion decreases, resulting in insufficient torsional strength of the trunnion's base portion.

The present invention aims to improve the strength of the base of the leg shaft of a tripod component.

The present invention, which has been made based on the above findings, comprises an outer joint member having track grooves extending in the joint axial direction at three locations in the circumferential direction, each of which has a pair of roller guideways arranged opposite each other in the joint circumferential direction, a body portion having a central hole, three trunnions protruding in the radial direction of the body portion, and an intermediate portion located between the body portion and the trunnions and having a curved cross section including the axis of the trunnions, the tripod member being made of steel, rollers attached to each of the trunnions, and an inner ring fitted to the trunnions and supporting the rollers so as to be freely rotatable, the rollers being rotated forward along the roller guideways. The tripod-type constant velocity universal joint is movable in the axial direction of the outer joint member, the roller and the inner ring form a roller unit that can swing with respect to the trunnion, the carbon content in the core of the tripod member is 0.23 to 0.44%, and a carburized hardened layer is provided on the surface of the trunnion. The tripod member is characterized in that a first region having a radius of curvature Ra in a cross section perpendicular to the joint axial direction, including the axis of the trunnion, and a second region having a radius of curvature Rb in a cross section in the joint axial direction, including the axis of the trunnion, are provided in the middle of the tripod member, with Ra > Rb.

This configuration reduces stress concentration in the first region where torque mainly acts. Therefore, it is possible to increase the torsional strength at the base of the leg shaft where tensile load acts repeatedly as torque is transmitted. As a result, by carburizing and quenching steel with a carbon content of 0.23% or more, the internal hardness of the tripod member is increased and the depth of the hardened layer on the surface is deepened, while the durability of the tripod member can be increased even in situations where there is concern that the torsional strength of the tripod member will decrease due to a decrease in toughness caused by an increase in internal hardness.

It is preferable to provide a connection region S between the first region and the second region, which smoothly connects both regions.

The pitch circle diameter of the roller guideway of the outer joint member is preferably PCD, and Ra/PCD ≥ 0.0850. By making Ra/PCD ≥ 0.0850 in this way, the thickness of the first region, i.e., the minimum distance t between the large diameter portion of the spline and the first region, can be increased. By increasing the thickness of the first region in this way, the strength of the base portion of the leg shaft, especially the fatigue strength, can be increased, even if the depth of the hardened layer increases and the toughness of the tripod member decreases.

In this case, it is preferable that t is the minimum distance from the large diameter portion of the spline formed on the inner peripheral surface of the body of the tripod member to the first region, and that t/PCD is 0.145 or greater.

The surface hardness of the tripod member's truss is preferably 653 HV or more. This increases the durability of the outer circumferential surface of the truss under high torque load, particularly the durability of the contact area with the inner circumferential surface of the inner ring.

The internal hardness of the tripod member is preferably 513 HV or more. By making the internal hardness 513 HV or more, the effective hardened layer depth required for the tripod member can be obtained.

The present invention makes it possible to improve the durability of the base of the leg shaft of the tripod component.

FIG. 2 is a cross-sectional view in the joint axial direction showing a double-roller type tripod constant velocity universal joint. 2 is a cross-sectional view taken along line K-K in FIG. 1 . 2 is a cross-sectional view taken along line LL in FIG. 1. 2 is a cross-sectional view showing a state in which the tripod type constant velocity universal joint of FIG. 1 has an operating angle. FIG. 4 is a cross-sectional view showing a hardened layer formed on a tripod member. FIG. 13 is a diagram showing the hardness distribution in the leg shaft of a conventional product. FIG. 13 is a diagram showing the hardness distribution at the leg shaft of the improved product. FIG. 4 is a front view of the tripod member as viewed from the joint axial direction. FIG. 2 is a side view (partially cross-sectional view) of a tripod member. 9 is a cross-sectional view taken along line MM in FIG. 8 . FIG. 1A is an enlarged cross-sectional view of a first region, and FIG. 1B is an enlarged cross-sectional view of a second region. FIG. 3 is an enlarged cross-sectional view of the first region and its vicinity of the tripod member in FIG. 2 .

An embodiment of a tripod-type constant velocity universal joint according to the present invention will be described with reference to Figures 1 to 12.

The tripod type constant velocity universal joint 1 of this embodiment shown in Figures 1 to 4 is of a double roller type. Note that Figure 1 is an axial cross-sectional view of a double roller type tripod type constant velocity universal joint, and Figure 2 is a cross-sectional view taken along line K-K in Figure 1. Figure 3 is a cross-sectional view taken along line L-L in Figure 1, and Figure 4 is an axial cross-sectional view showing a tripod type constant velocity universal joint when an operating angle is taken. Note that in the following description, the joint axial direction and joint circumferential direction respectively refer to the axial direction and circumferential direction of the tripod type constant velocity universal joint when the operating angle is set to 0°.

As shown in Figures 1 and 2, the tripod type constant velocity universal joint 1 is mainly composed of an outer joint member 2, a tripod member 3 as an inner joint member, and a roller unit 4 as a torque transmission member. The outer joint member 2 is cup-shaped with one end open, and three linear track grooves 5 extending in the joint axial direction are formed on the inner peripheral surface at equal intervals in the joint circumferential direction. Each track groove 5 is arranged opposite to the joint circumferential direction of the outer joint member 2, and has a roller guide surface 6 extending in the joint axial direction. The tripod member 3 and roller unit 4 are housed inside the outer joint member 2.

The tripod member 3 has a body 31 (trunnion body) having a central hole 30, three leg shafts 32 (trunnion journals) protruding radially from three equal positions in the joint circumferential direction on the outer peripheral surface of the body 31, and an intermediate portion 33 connecting the outer peripheral surface of the body 31 and the outer peripheral surface of the leg shafts 32. The tripod member 3 is connected to the shaft 8 so as to be able to transmit torque by fitting a male spline 81 formed on the shaft 8 as an axis to a female spline 34 formed in the central hole 30 of the trunnion body 31. The tripod member 3 is fixed to the shaft 8 in the joint axial direction by engaging an end face on one side of the joint axial direction of the tripod member 3 with a shoulder portion 82 provided on the shaft 8 and engaging a retaining ring 10 attached to the tip of the shaft 8 with an end face on the other side of the joint axial direction of the tripod member 3.

The roller unit 4 is mainly composed of an outer ring 11, which is a circular roller centered on the axis of the leg axle 32, an inner ring 12, which is a circular roller arranged on the inner diameter side of the outer ring 11 and fitted onto the leg axle 32, and a number of needle rollers 13 interposed between the outer ring 11 and the inner ring 12. The roller unit 4 is housed in the track groove 5 of the outer joint member 2. The roller unit 4, which is composed of the outer ring 11, the inner ring 12, and the needle rollers 13, is structured so that they cannot be separated by the washers 14 and 15.

In this embodiment, the outer peripheral surface 11a (see FIG. 2) of the outer ring 11 is a convex curved surface whose generating line is a circular arc having a center of curvature on the axis of the leg shaft 32. The outer peripheral surface 11a of the outer ring 11 is in angular contact with the roller guideway 6.

The needle rollers 13 are arranged to roll freely between the cylindrical inner peripheral surface of the outer ring 11, which serves as the outer raceway surface, and the cylindrical outer peripheral surface of the inner ring 12, which serves as the inner raceway surface.

The outer peripheral surface of each leg shaft 32 of the tripod member 3 has a straight shape in the axial direction of the leg shaft 32 in a cross section in any direction including the axis of the leg shaft 32. Also, as shown in FIG. 3, the outer peripheral surface of the leg shaft 32 has a substantially elliptical shape in a cross section perpendicular to the axis of the leg shaft 32. The outer peripheral surface of the leg shaft 32 contacts the inner peripheral surface 12a of the inner ring 12 in a direction perpendicular to the joint axis direction, i.e., in the direction of the major axis a. In the joint axis direction, i.e., in the direction of the minor axis b, a gap m is formed between the outer peripheral surface of the leg shaft 32 and the inner peripheral surface 12a of the inner ring 12.

As shown in Figures 1 and 2, the intermediate portion 33 between the body portion 31 and the leg shaft 32 of the tripod member 3 is formed to draw a concave curve in any cross section including the axis of the leg shaft 32.

The inner peripheral surface 12a of the inner ring 12 forms a convex arc shape in any cross section including the axis of the inner ring 12. Because the cross section of the leg axle 32 is substantially elliptical as described above, and a predetermined gap m is provided between the leg axle 32 and the inner ring 12, the inner ring 12 can oscillate with respect to the leg axle 32. As described above, the inner ring 12 and the outer ring 11 are assembled via the needle rollers 13 so as to be relatively rotatable, so the outer ring 11 can oscillate with respect to the leg axle 32 together with the inner ring 12. In other words, the axes of the outer ring 11 and the inner ring 12 can tilt with respect to the axis of the leg axle 32 within a plane including the axis of the leg axle 32 (see FIG. 4).

As shown in Figure 4, when the tripod-type constant velocity universal joint 1 rotates through an operating angle, the axis of the tripod member 3 is inclined relative to the axis of the outer joint member 2, but because the roller unit 4 is swingable, it is possible to prevent the outer ring 11 and the roller guideway 6 from intersecting at an angle. As a result, the outer ring 11 rolls horizontally relative to the roller guideway 6, reducing induced thrust and sliding resistance, and achieving low vibration in the tripod-type constant velocity universal joint 1.

As already mentioned, the cross section (transverse section) of the leg axle 32 is approximately elliptical, and the cross section (longitudinal section) of the inner peripheral surface 12a of the inner ring 12 is an arc-shaped convex cross section, so that the outer peripheral surface of the leg axle 32 on the torque load side and the inner peripheral surface 12a of the inner ring 12 come into point contact or contact over a small area close to point contact. This reduces the force that tends to tilt the roller unit 4, improving the stability of the posture of the outer ring 11.

The tripod member 3 described above is manufactured from steel material through the following main processes: forging (cold forging) → machining (turning) → broaching the spline 34 → heat treatment → grinding the outer circumferential surface of the leg shaft 32. The outer circumferential surface of the leg shaft 32 can also be finished by cutting hardened steel instead of the grinding process. In addition, a spheroidizing annealing process and a bonderizing process can be added before cold forging. If there is no problem with the hammering forgeability during cold forging due to circumstances such as the use of a material with a low carbon content, the spheroidizing annealing process can be omitted. Carburizing, quenching, and tempering are performed as heat treatment.

Figure 5 is a cross-sectional view showing the hardened layer 16 formed by heat treatment of the tripod member 3. The hardened layer 16 is formed by hardening the carburized layer by quenching. The hardened layer 16 is formed on the entire surface of the tripod member 3, including the outer circumferential surface of the leg axle 32, the outer circumferential surface of the body 31, the surface of the intermediate portion 33, and the surface of the female spline 34. In the completed tripod member 3, the outer circumferential surface of the leg axle 32 is finished by grinding (or hardened steel cutting), so the depth of the hardened layer 16 on the outer circumferential surface of the leg axle 32 is shallower than other areas by the amount of the machining allowance due to grinding, etc. Note that this machining allowance is usually small, about 0.1 mm, so in Figure 5 the thickness of the hardened layer 16 is drawn uniformly on the entire surface.

As already mentioned, in a double roller type tripod constant velocity universal joint, as shown in FIG. 3, the outer peripheral surface of the trunnion 32 and the inner peripheral surface 12a of the inner ring 12 make point contact or near-point contact in region X on the torque load side, which creates the problem that the surface pressure at the contact point becomes high when a high torque load is applied. If the surface pressure is excessive, it may affect the durability of the trunnion 32 at the contact point X.

To solve this problem, the inventors conducted the following verification.

In general, the tripod member 3 is forged from chromium-molybdenum steel, a type of case-hardened steel, and then carburized, quenched, and tempered as a heat treatment to form a hardened layer 16 on the surface. Figure 6 shows the hardness distribution from the surface to the core of the leg shaft 32 when a conventional tripod member 3 material (e.g., chromium-molybdenum steel of JIS G4052, equivalent material with a carbon content of less than about 0.23%) is used and carburized, quenched, and tempered (quenching temperature 860°C, tempering temperature 180°C). In this case, as is clear from Figure 6, the surface hardness exceeds 513HV, but the hardness falls below 513HV in a very shallow area from the surface. Therefore, if an excessive torque is applied, it affects the durability of the leg shaft 32 at the contact portion. Therefore, in order to solve the above problem, it is necessary to form the hardened layer 16 as deep as possible.

The effective hardened layer depth means the distance from the surface of the steel material to the position of the limit hardness. According to JIS G0557, the limit hardness of the effective hardened layer is 550 HV, but it is also stipulated that "if the hardness at a position three times the distance from the surface of the hardened layer exceeds 450 HV Vickers hardness, a limit hardness exceeding 550 HV may be used by agreement between the parties." In this embodiment, as described later, the internal hardness (hardness of the unhardened area) of the tripod member 3 is 513 HV or more, so in this embodiment, the limit hardness of the effective hardened layer depth is stipulated as 600 HV in consideration of the above exception. Note that the harder the hardened layer 16, the better in terms of durability of the leg shaft 32, so it is preferable to stipulate the limit hardness of the effective hardened layer depth to 653 HV or more.

The easiest way to deepen the hardened layer 16 is to increase the depth of the carburized layer, but forming a deep carburized layer requires a huge amount of carburizing time, which leads to high manufacturing costs. It is also possible to use steel with a high carbon content, such as carbon steel for machine construction such as S50C to S55C, as the raw material, and change the heat treatment method to high-frequency hardening, which allows for deeper hardening than carburized hardening. However, in this case, the material becomes harder as the carbon content increases, which increases the processing load when forging the tripod member 3, and this leads to problems such as the need for larger forging equipment.

After considering the above, the inventors verified the effectiveness of using case-hardened steel with a higher carbon content than conventional steels while keeping the carburizing and quenching/tempering conditions the same as conventional steels. Figure 7 shows the hardness distribution when carburizing, quenching, and tempering are performed using chromium-molybdenum steel with a carbon content of approximately 0.34% as the material. The quenching temperature is 850°C, and the tempering temperature is 180°C. The horizontal axis in Figure 7 (depth from the surface) is shown on the same scale as Figure 6.

As is clear from the results in Figure 7, it was found that by increasing the carbon content of the case-hardened steel, the depth of the hardened layer 16 can be increased as intended. It can also be seen that as a result of this increased depth of the hardened layer 16, the internal hardness becomes 513 HV or more. On the other hand, because the hardness (internal hardness) of the core after carburizing, quenching and tempering reaches about 550 HV, there is a risk that the toughness of the leg shaft 32 will decrease, and the repeated fatigue strength of the tripod member 3 will decrease. Measures to address this issue will be described later.

In the above description, a case where a material equivalent to a carbon content of about 0.34% is used as the material for the tripod member 3 is exemplified, but the type of material that can be used is not limited. For example, in the case of chromium-molybdenum steel, in addition to SCM435, SCM440, etc. can be used. Also, so-called H-steel (e.g., SCM435H, SCM440H, etc.: specified in JIS G4052) with guaranteed hardenability can be used. In the case of case-hardened steel, other types of steel can be used, and for example, chromium steel (e.g., SCr435, SCr440, etc.) specified in JIS G4053 can be used as the material. As for chromium steel, for example, H-steel such as SCr435H, SCr440H, etc. can be used. Not limited to case-hardened steel such as chromium-molybdenum steel or chromium steel, carbon steel for machine structure such as S10C to S35C (JIS G40
51) may also be used as material.

Considering formability during cold forging of the tripod member 3, it is preferable to use steel with a carbon content of 0.44% or less. However, if formability during forging is not an issue, such as when hot forging, steel with a carbon content exceeding 0.44% can also be used. Case-hardened steel with a carbon content of 1% or less will not cause any particular problems during hot forging.

As already mentioned, it was found that the improved product described above had a problem with the strength of the base portion (middle portion 33) of the leg shaft 32. The cause of this is presumably that the toughness of the tripod member 3 is reduced by increasing the hardness of the entire tripod member 3 from the surface to the deep regions, which reduces the fatigue strength of the tripod member 3 in the middle portion 33 where tensile loads are repeatedly applied with torque transmission, affecting the strength of the middle portion 33. If an attempt is made to solve this problem by reviewing the materials and heat treatment method, this could reduce the durability of the contact portion X of the leg shaft 32, and it is desirable to solve the problem from a different perspective.

Based on the above verification, in this invention, we decided to reconsider the shape of the tripod member 3 in order to improve the strength of the base part of the leg shaft 32.

As shown in Figures 8 to 10, the intermediate portion 33 is formed so that the cross section including the axis of the leg axle 32 is a concave curve around the entire circumference of the leg axle 32. As shown in Figure 11, the intermediate portion 33 is formed in an arc shape with a radius of curvature Ra in a cross section including the axis of the leg axle 32 and perpendicular to the joint axis direction (the major axis direction of the ellipse formed by the cross section of the leg axle 32), and is formed in an arc shape with a radius of curvature Rb in a cross section including the axis of the leg axle 32 and in the joint axis direction (the minor axis direction of the ellipse formed by the cross section of the leg axle 32). The radius of curvature Ra is greater than the radius of curvature Rb (Ra>Rb).

As shown in FIG. 10, the first region P having a radius of curvature Ra and the second region Q having a radius of curvature Rb in the intermediate portion 33 are each formed with a width in the circumferential direction of the leg shaft 32. For example, as shown in FIG. 10, the first region P can be formed in a partial circumferential region of the leg shaft 32 centered on a plane perpendicular to the joint axial direction including the axis of the leg shaft 32. The second region Q can be formed in a partial circumferential region of the leg shaft 32 centered on a plane in the joint axial direction including the axis of the leg shaft 32. In the circumferential direction of the leg shaft 32, two first regions P are arranged opposite each other in the direction perpendicular to the joint axial direction, and two second regions Q are arranged opposite each other in the joint axial direction.

As shown in FIG. 10, the intermediate portion 33 is sandwiched between the adjacent first region P and second region Q in the circumferential direction of the truss axle 32, and has a connection region S that smoothly connects to both the first region P and the second region Q. In this connection region S, the radius of curvature gradually changes in the circumferential direction of the truss axle 32.

In this way, the intermediate portion 33 is formed of the first region P, the second region Q, or the connection region S. Each of these regions P, Q, and S can be formed of a single arc in a vertical cross section including the axis of the leg axle 32, or can be formed of multiple arcs with different radii of curvature. In the latter case, the shape of the intermediate portion 33 is determined so that Ra>Rb, with the minimum radius of curvature in the cross section including the axis of the leg axle 32 being Ra in the first region P and the maximum radius of curvature in the cross section including the axis of the leg axle 32 being Rb. It is also possible to form one or two of the regions P, Q, and S as a single arc, and the remaining regions as multiple arcs.

In this way, by making the radius of curvature Ra of the first region P larger than the radius of curvature Rb of the second region Q, it is possible to reduce stress concentration in the first region P where torque mainly acts. Therefore, it is possible to increase the torsional strength at the base of the leg shaft 32 where tensile load acts repeatedly with torque transmission. As a result, by performing carburizing and quenching on case-hardened steel with a carbon content of 0.23% or more, it is possible to increase the internal hardness of the tripod member 3 and deepen the depth of the hardened layer on the surface, while also increasing the durability of the tripod member 3 even in situations where there is concern about a decrease in the torsional strength of the tripod member due to a decrease in toughness caused by an increase in internal hardness.

The second region Q also has a low contribution to torque transmission. Therefore, by reducing the radius of curvature Rb of the second region Q, more material is removed during processing, making it possible to reduce the weight of the tripod member 3 while avoiding a decrease in strength. This effect can be more pronounced by making the angle β of the formation range of the second region Q centered on the axis of the truss 32 larger than the angle α of the formation range of the first region P centered on the axis of the truss 32, as shown in FIG. 10.

In a double-roller type tripod member 3 such as this embodiment, the needle rollers 13 do not roll on the outer circumferential surface of the leg axle 32, so there is no need to provide a reduced-weight portion (recessed portion) in the area from the outer circumferential surface of the leg axle 32 to the surface of the intermediate portion 33 to avoid interference with the needle rollers 13. Therefore, no edges are formed due to the recessed portion in the area from the surface of the intermediate portion 33 to the outer circumferential surface of the leg axle 32, and the area from the intermediate portion 33 to the outer circumferential surface of the leg axle 32 is smoothly continuous. Therefore, the effect of reducing stress concentration by providing a difference between the radii of curvature Ra and Rb can be fully obtained.

The radius of curvature Ra in the first region P of the intermediate portion 33 is preferably set to Ra/PCD≧0.0850, where PCD (see FIG. 2) is the pitch circle diameter of the roller guideway 6 of the outer joint member 2. Also, t (see FIG. 5) is the minimum distance from the large diameter portion 34a of the spline 34 formed on the inner peripheral surface of the body portion 31 of the tripod member 3 to the first region P, and t/PCD≧0.145. Note that PCD, Ra, and t are all in the same unit (mm).

The reasons for setting these numerical ranges are as follows:

12 is an enlarged cross-sectional view of the middle part 33 (first region P) of the tripod member 3. As shown by the solid line in FIG. 12, the inner diameter side of the first region P is smoothly connected to the outer circumferential surface of the body 31 by drawing a tangent line. On the other hand, the outer diameter side of the first region P is connected to the outer circumferential surface of the leg shaft 32 via a small step Z. This step Z is caused by the outer circumferential surface of the leg shaft 32 receding by the grinding allowance when the outer circumferential surface of the leg shaft 32 is ground after the cold forging of the tripod member 3. As shown by the two-dot chain line, when the radius of curvature of the middle part 33' (first region) is increased, the arc-shaped middle part 33' reaches the grinding area G before grinding on the outer diameter side of the middle part 33', and the grinding allowance Y becomes large. The increase in the grinding allowance Y has a negative effect on the grinding accuracy. In order to prevent a decrease in grinding accuracy, the conventional product has a setting of R/PCD<0.0850. Note that the "conventional product" here refers to a product in which the middle portion 33 has a uniform radius of curvature R over its entire circumference.

In the present invention, since Ra/PCD≧0.0850, the thickness of the first region P, i.e., the minimum distance t (thickness in FIG. 12) between the large diameter portion 34a of the spline 34 (see FIG. 5) and the first region P, can be increased. Specifically, t/PCD≧0.145 can be achieved. By increasing the thickness of the first region P in this way, even if the depth of the hardened layer 16 is increased and the toughness of the tripod member 3 is reduced, the strength, particularly the fatigue strength, of the first region P of the intermediate portion 33 of the truss 32 can be increased. This makes it possible to increase the torsional strength of the truss 32 and improve the design freedom of the tripod member 3.

Increasing the radius of curvature Ra of the first region P in this way increases the grinding allowance Y on the outer diameter side of the first region P, but through verification by the inventor, it has been confirmed that as long as Ra/PCD is in the range of 0.20, there is no adverse effect on the grinding accuracy when grinding the outer peripheral surface of the leg shaft 32. Therefore, the upper limit of Ra/PCD is preferably 0.20. In other words, it is preferable to set it to 0.0850≦Ra/PCD≦0.20. Also, if the value of t/PCD is too large, the tripod member 3 will become unnecessarily large, resulting in an increase in weight, so the upper limit of the value of t/PCD is preferably 0.20 (t/PCD≦0.20).

The radius of curvature Rb of the second region Q is preferably in the range of 0.0550≦Rb/PCD≦0.0820. As described above, when Ra>Rb, more material is removed during processing, but when Ra and Rb are close to each other, it is difficult to obtain a clear material reduction effect. Therefore, it is preferable that Rb/PCD≦0.0820. Furthermore, if Rb/PCD is too small, when the second region Q is formed by forging, the formability may deteriorate and it may not be possible to form it into the specified shape. Therefore, it is preferable that 0.0550≦Rb/PCD.

The above-described embodiments of the present invention can also be applied to double-roller type tripod constant velocity universal joints having other configurations.

For example, the outer peripheral surface of the trunnion 32 can be formed as a convex curved surface (e.g., a convex arc-shaped cross section), and the inner peripheral surface 12a of the inner ring 12 can be formed as a cylindrical surface. Also, the outer peripheral surface of the trunnion 32 can be formed as a convex curved surface (e.g., a convex arc-shaped cross section), and the inner peripheral surface 12a of the inner ring 12 can be formed as a concave spherical surface that fits with the outer peripheral surface of the trunnion. In this case, by providing flanges on both ends of the inner diameter of the outer ring, the washers 14, 15 can be made unnecessary.

The above-described tripod-type constant velocity universal joint 1 is not limited to application in drive shafts of automobiles, but can be widely used in power transmission paths of automobiles, industrial equipment, etc.

1 Tripod type constant velocity universal joint 2 Outer joint member 3 Tripod member 4 Roller unit 5 Track groove 6 Roller guide surface 8 Shaft
11. Roller (outer ring)
12 Inner ring 13 Needle roller 16 Hardened layer 30 Center hole 31 Body portion 32 Leg shaft 33 Middle portion 34 Female spline P First region Q Second region

Claims (5)

an outer joint member provided with track grooves extending in a joint axial direction at three locations in a circumferential direction, each track groove having a pair of roller guide surfaces disposed opposite to each other in the joint circumferential direction;
a tripod member made of steel, the tripod member including a body having a central hole, three leg shafts protruding in a radial direction of the body, and an intermediate portion located between the body and the leg shafts and having a curved cross section including the axis of the leg shaft;
A roller attached to each of the leg shafts;
an inner ring that is fitted onto the trunnion and rotatably supports the roller;
the rollers are movable in the axial direction of the outer joint member along the roller guideway,
the roller and the inner ring constitute a roller unit that is swingable relative to the trunnion;
The carbon content in the core of the tripod member is 0.23 to 0.44%,
In a tripod type constant velocity universal joint in which a carburized hardened layer is provided on the surface of the leg shaft,
a first region having a radius of curvature Ra in a cross section perpendicular to a joint axial direction and including an axis of the trunnion, and a second region having a radius of curvature Rb in a cross section in the joint axial direction and including the axis of the trunnion, the second region satisfying Ra>Rb ;
a step Z is provided between the first region of the intermediate portion and the trunnion, the step Z being reduced in diameter from the first region in the trunnion radial direction and connected to an outer circumferential surface of the trunnion;
A tripod-type constant velocity universal joint , wherein a pitch circle diameter PCD of a roller guideway of the outer joint member satisfies 0.085≦Ra/PCD≦0.20. The tripod type constant velocity universal joint according to claim 1, in which a connection region S is provided between the first region and the second region, smoothly connecting both regions. 2. The tripod type constant velocity universal joint according to claim 1 , wherein t is a minimum distance from a large diameter portion of a spline formed on an inner peripheral surface of a body portion of the tripod member to the first region, and t/PCD is greater than or equal to 0.145. 4. The tripod type constant velocity universal joint according to claim 1 , wherein the surface hardness of the trunnions of the tripod members is 653 HV or more. 5. The tripod type constant velocity universal joint according to claim 1 , wherein the internal hardness of said tripod members is 513 HV or more.

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